diff --git a/changelog.d/fixes/13628-grok46-default-effort.md b/changelog.d/fixes/13628-grok46-default-effort.md
new file mode 100644
index 0000000000..1ee9f2f810
--- /dev/null
+++ b/changelog.d/fixes/13628-grok46-default-effort.md
@@ -0,0 +1,2 @@
+- fix(providers): restore grok-4.6/4.5 default reasoning effort so requests without an explicit effort keep reasoning enabled (#13628)
+- fix(registry): declare supportedThinkingEfforts on claude-opus-5 and claude-fable-5 across the anthropic/claude/claude-web/ghe-copilot/github registries (#13628)
diff --git a/config/quality/eslint-suppressions.json b/config/quality/eslint-suppressions.json
index 5ffac46924..f1005c77c1 100644
--- a/config/quality/eslint-suppressions.json
+++ b/config/quality/eslint-suppressions.json
@@ -21,7 +21,7 @@
   },
   "open-sse/config/providers/registry/claude/index.ts": {
     "@typescript-eslint/no-unused-vars": {
-      "count": 7
+      "count": 6
     }
   },
   "open-sse/config/providers/registry/vertex/index.ts": {
diff --git a/open-sse/config/providers/registry/grok-cli/index.ts b/open-sse/config/providers/registry/grok-cli/index.ts
index e65ced0e76..ffdcc7cd62 100644
--- a/open-sse/config/providers/registry/grok-cli/index.ts
+++ b/open-sse/config/providers/registry/grok-cli/index.ts
@@ -26,6 +26,7 @@ export const grok_cliProvider: RegistryEntry = {
       name: "Grok 4.6",
       contextLength: 500000,
       supportsReasoning: true,
+      supportedThinkingEfforts: ["low", "medium", "high"],
       toolCalling: true,
       targetFormat: "openai-responses",
       unsupportedParams: ["presencePenalty", "frequencyPenalty", "logprobs", "topLogprobs"],
@@ -35,6 +36,7 @@ export const grok_cliProvider: RegistryEntry = {
       name: "Grok 4.5",
       contextLength: 500000,
       supportsReasoning: true,
+      supportedThinkingEfforts: ["low", "medium", "high"],
       toolCalling: true,
       targetFormat: "openai-responses",
       unsupportedParams: ["presencePenalty", "frequencyPenalty", "logprobs", "topLogprobs"],
diff --git a/open-sse/executors/grok-cli.ts b/open-sse/executors/grok-cli.ts
index fc37b23ce2..b9e004a538 100644
--- a/open-sse/executors/grok-cli.ts
+++ b/open-sse/executors/grok-cli.ts
@@ -127,13 +127,18 @@ function normalizeGrokBuildReasoning(
   model: string
 ): Record<string, unknown> | null {
   const reasoning = asRequestRecord(value);
+  // Capture BEFORE stripping: an explicit (but unsupported/invalid, e.g. "none"/"off"/
+  // "xhigh") effort must still count as an explicit off-switch below — only the true
+  // ABSENCE of an effort key gets the model default. Restores the #7358 behavior the
+  // 4.6 default accidentally regressed: without this, every explicit "none"/"off" from
+  // grok-cli got silently promoted to "high", leaving no way to disable reasoning.
   const hasExplicitEffort = Object.prototype.hasOwnProperty.call(reasoning, "effort");
   if (!GROK_BUILD_REASONING_EFFORT_SET.has(String(reasoning.effort))) {
     delete reasoning.effort;
   }
   if (model === "grok-composer-2.5-fast") {
     delete reasoning.effort;
-  } else if (model === "grok-4.5" && !hasExplicitEffort) {
+  } else if ((model === "grok-4.5" || model === "grok-4.6") && !hasExplicitEffort) {
     reasoning.effort = GROK_BUILD_DEFAULT_REASONING_EFFORT;
   }
   return Object.keys(reasoning).length > 0 ? reasoning : null;
diff --git a/tests/unit/grok-cli-reasoning-strip-6288.test.ts b/tests/unit/grok-cli-reasoning-strip-6288.test.ts
index f9e066cbab..8035fd022c 100644
--- a/tests/unit/grok-cli-reasoning-strip-6288.test.ts
+++ b/tests/unit/grok-cli-reasoning-strip-6288.test.ts
@@ -96,6 +96,9 @@ test("grok-cli preserves explicit store and de-duplicates encrypted reasoning in
 
   assert.equal(out.store, true);
   assert.deepEqual(out.include, ["reasoning.encrypted_content"]);
+  // An explicit (but unsupported) effort like "xhigh" is still an EXPLICIT effort key —
+  // it gets stripped, not defaulted. Only the true absence of an "effort" key falls back
+  // to the model default. This is the off-switch #7358 relied on and #13628 regressed.
   assert.equal("reasoning" in out, false);
 });
 
@@ -113,3 +116,94 @@ test("grok-cli preserves an explicit Responses reasoning summary", () => {
 
   assert.deepEqual(out.reasoning, { summary: "concise", effort: "high" });
 });
+
+test("grok-4.6 applies default high when client omits effort", async () => {
+  const executor = new GrokCliExecutor();
+  const body = {
+    model: "grok-4.6",
+    input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+  };
+
+  const transformed = executor.transformRequest(
+    "grok-4.6",
+    body,
+    false,
+    {} as Record<string, unknown>
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(transformed.reasoning, { effort: "high" });
+});
+
+test("grok-4.6 preserves an explicit supported effort", async () => {
+  const executor = new GrokCliExecutor();
+  const body = {
+    model: "grok-4.6",
+    input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning: { effort: "medium", summary: "auto" },
+  };
+
+  const transformed = executor.transformRequest(
+    "grok-4.6",
+    body,
+    false,
+    {} as Record<string, unknown>
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(transformed.reasoning, { effort: "medium", summary: "auto" });
+});
+
+test("grok-4.6 strips an explicit but unsupported xhigh (no default restore — an explicit effort key is an explicit choice)", async () => {
+  const executor = new GrokCliExecutor();
+  const body = {
+    model: "grok-4.6",
+    input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning: { effort: "xhigh" },
+    reasoning_effort: "xhigh",
+  };
+
+  const transformed = executor.transformRequest(
+    "grok-4.6",
+    body,
+    false,
+    {} as Record<string, unknown>
+  ) as Record<string, unknown>;
+
+  assert.equal("reasoning_effort" in transformed, false);
+  assert.equal("reasoning" in transformed, false);
+});
+
+test("grok-4.6 keeps an explicit none/off as a real off-switch (no default restore)", async () => {
+  const executor = new GrokCliExecutor();
+  const body = {
+    model: "grok-4.6",
+    input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning: { effort: "none" },
+  };
+
+  const transformed = executor.transformRequest(
+    "grok-4.6",
+    body,
+    false,
+    {} as Record<string, unknown>
+  ) as Record<string, unknown>;
+
+  assert.equal("reasoning" in transformed, false);
+});
+
+test("grok-4.5 keeps an explicit none/off as a real off-switch (no default restore)", async () => {
+  const executor = new GrokCliExecutor();
+  const body = {
+    model: "grok-4.5",
+    input: [{ role: "user", content: [{ type: "input_text", text: "hi" }] }],
+    reasoning: { effort: "off" },
+  };
+
+  const transformed = executor.transformRequest(
+    "grok-4.5",
+    body,
+    false,
+    {} as Record<string, unknown>
+  ) as Record<string, unknown>;
+
+  assert.equal("reasoning" in transformed, false);
+});